RF systems are capable of communicating a large amount of information data in a non-contact state and therefore have been used instead of bar code systems in the application fields such as accounting, prepayment, security management or control, article control or distribution control and traceability (Patent Document 1).
In the application fields of accounting, prepayment and security management, it is intended to restrict a communicable range of the RF systems in view of their configuration upon use. For this reason, in these application fields, there has been employed an electromagnetic induction system mainly using a frequency of 13.56 MHz or the like. On the other hand, in the application fields of article control or distribution control, traceability and the like, there has been used a radio wave system using a UHF band (840 to 958 MHz, 2.45 GHz) which is capable of remote communication at a distance of several meters or more. It is expected that in the future, the radio wave system will have a further progress.
An RF tag used in the RF system operated in a UHF band has such a structure that an IC chip for storing and processing data and an antenna for communicating the data with an outside are mounted on a flat thin base sheet formed of a resin, a paper or the like. The RF tag is attached to an article to be managed, and communicates necessary information data with a reader/writer using a radio wave.
The size of the antenna may be substantially determined according to a frequency used, i.e., a wavelength λ, of the radio wave. The tag used in a UHF band has a size of about 15 mm in length and about 145 mm in width. Therefore, the RF tag is necessarily incapable of being attached to a small article having a dimension smaller than the above size. As a result, such an RF tag can be used only in the limited applications. To solve the problems, there has been proposed an RF tag comprising a dipole antenna having a folded tip end in which an inductance portion is disposed (Patent Document 2).
Also, the RF tag is disposed close to or attached to a dielectric material such as a resin, a metal, a paper, water, etc., upon use. In this case, it is known that frequency characteristics of a feeding point impedance of the antenna are fluctuated so that reading characteristics of the RF tag are deteriorated. To solve this problem, there have been proposed the above method described in Patent Document 1 and a method using an antenna having such a configuration that the antenna is sandwiched between plastic materials (Patent Document 3).
On the other hand, a hexagonal ferrite has been proposed as a magnetic material that can be used in a UHF band (Patent Document 4).